Printed circuit boards of diverse size are extensively used in the manufacture of innumerable electrical devices. These boards have circuit paths formed on one or both sides and electrical components are mounted on and connected to the circuit paths. The circuit components are usually secured to the circuit paths by a soldering operation such as wave soldering. These printed circuit boards also find use as elements of connectors, such as back plane connectors, extensively used in construction of telephone equipment. One type of back plane connector consists of a small printed circuit board having parallel conductor paths running across both sides of the board to edge contact pads which are overlayed with terminals projecting from sockets mounted on the opposite edges of the boards. The terminals are bonded to the contact pads with solder that may be applied by moving the boards through a narrow solder wave. In the application of solder waves to these connector boards, great care must be exercised to be sure that the solder is only applied to the finite areas of the overlayed terminals and contact pads. In the manufacture of many other printed circuit boards there is often a requirement that the solder be only applied at specific definitive solder sites.
Mass production of printed circuit boards requires automated facilities for applying solder to a number of sites on the board to establish solder connections between the printed circuit paths and the components or other elements mechanically mounted on the board. One common mass soldering expedient contemplates the generation of a wave of molten solder and then conveying the circuit boards in such manner as to engage the undersides of the boards with the wave of solder to deposit solder at the desired sites. In many instances the application of solder must be restricted to specific sites because application of solder to other areas on the board may result in the damage of heat sensitive components mounted on the board or the flow of solder into plated through holes that are to be subsequently utilized to mount additional components. In the manufacture of the previously mentioned printed circuit board connectors, there is a requirement that the solder application be confined to those areas containing the terminals and the overlying contact pads formed on the board.
Selective application of solder to designated solder sites of the circuit board may be accomplished by applying solder resist coatings to those areas which are not to receive solder. This expedient results in added manufacturing cost, in that time must be taken to apply the resist, remove the resist, and clean the circuit board. Another expedient contemplates securing a mask to the board leaving those areas to receive the solder exposed during subsequent movement of the board through a solder wave. Again, added manufacturing steps are required, increasing the cost of the soldering application. The masks are often expensive to construct and unless firmly secured to the board there is a tendency for the solder to creep under the mask into areas that should be devoid of solder.
In instances where the selected sites on the circuit board are arranged in a row, an adjustable weir may be used to set the width of the solder wave so that the solder is only applied to the selected row of solder sites as the board is conveyed through the solder wave. An example of such an apparatus is shown in U.S. Pat. No. 3,604,611, issued Sept. 14, 1971, to J. Lamberty, wherein a solder wave is generated and confined between a pair of vertically extending extending side plates one of which is adjustable to control the width of the wave, and hence the width of the solder applied to a circuit board that is moved through the solder wave by a conveyor.
Another problem that exists when solder is to be applied to selected sites on a printed circuit board, resides in the need to maintain solder sites in the solder wave for times sufficient to deposit coatings of solder of adequate thickness so as to establish good solder bonds. In order to meet this problem, solder wave equipments have been developed wherein a solder wave is flowed as a stream along an inclined channel in a direction opposite to the direction of movement of the conveyed printed circuit board through the stream. One such system is shown in U.S. Pat. No. 3,039,185, issued June 19, 1962, to W. L. Oates, wherein a solder wave is generated and flowed through an inclined channel having a number of transverse ribs which function to form the wave into a series of undulating crests through which the printed circuit board is conveyed at an angle corresponding to the angle of the incline.
It will be noted that many of the prior art devices depend on a conveyor for advancing the circuit boards through a solder wave. Due to the aggregrate of tolerances required in the construction of parts for the conveyor there are inherent mechanical limitations in providing a conveyor that is capable of precisely positioning a board as the board is passed through the solder wave. Further, in wave solder systems utilizing adjustable channel plates to control the width of the solder wave, an additional problem exists in that the conveyor speeds must be limited otherwise there is spillage over the sides of the channel plates onto sections of the printed circuit board that are required to be free of solder. In these mass soldering operations, concern must also be given to the control of the heat of the applied solder which may be detrimental to heat sensitive components mounted on other areas of the circuit board.